1Linker Script implementation notes and policy
2=============================================
3
4LLD implements a large subset of the GNU ld linker script notation. The LLD
5implementation policy is to implement linker script features as they are
6documented in the ld `manual <https://sourceware.org/binutils/docs/ld/Scripts.html>`_
7We consider it a bug if the lld implementation does not agree with the manual
8and it is not mentioned in the exceptions below.
9
10The ld manual is not a complete specification, and is not sufficient to build
11an implementation. In particular some features are only defined by the
12implementation and have changed over time.
13
14The lld implementation policy for properties of linker scripts that are not
15defined by the documentation is to follow the GNU ld implementation wherever
16possible. We reserve the right to make different implementation choices where
17it is appropriate for LLD. Intentional deviations will be documented in this
18file.
19
20Symbol assignment
21~~~~~~~~~~~~~~~~~
22
23A symbol assignment looks like:
24
25::
26
27  symbol = expression;
28  symbol += expression;
29
30The first form defines ``symbol``. If ``symbol`` is already defined, it will be
31overridden. The other form requires ``symbol`` to be already defined.
32
33For a simple assignment like ``alias = aliasee;``, the ``st_type`` field is
34copied from the original symbol. Any arithmetic operation (e.g. ``+ 0`` will
35reset ``st_type`` to ``STT_NOTYPE``.
36
37The ``st_size`` field is set to 0.
38
39SECTIONS command
40~~~~~~~~~~~~~~~~
41
42A ``SECTIONS`` command looks like:
43
44::
45
46  SECTIONS {
47    section-command
48    section-command
49    ...
50  } [INSERT [AFTER|BEFORE] anchor_section;]
51
52Each section-command can be a symbol assignment, an output section description,
53or an overlay description.
54
55When the ``INSERT`` keyword is present, the ``SECTIONS`` command describes some
56output sections which should be inserted after or before the specified anchor
57section. The insertion occurs after input sections have been mapped to output
58sections but before orphan sections have been processed.
59
60In the case where no linker script has been provided or every ``SECTIONS``
61command is followed by ``INSERT``, LLD applies built-in rules which are similar
62to GNU ld's internal linker scripts.
63
64- Align the first section in a ``PT_LOAD`` segment according to ``-z noseparate-code``,
65  ``-z separate-code``, or ``-z separate-loadable-segments``
66- Define ``__bss_start``, ``end``, ``_end``, ``etext``, ``_etext``, ``edata``, ``_edata``
67- Sort ``.ctors.*``/``.dtors.*``/``.init_array.*``/``.fini_array.*`` and PowerPC64 specific ``.toc``
68- Place input ``.text.*`` into output ``.text``, and handle certain variants
69  (``.text.hot.``, ``.text.unknown.``, ``.text.unlikely.``, etc) in the precense of
70  ``-z keep-text-section-prefix``.
71
72Output section description
73~~~~~~~~~~~~~~~~~~~~~~~~~~
74
75The description of an output section looks like:
76
77::
78
79  section [address] [(type)] : [AT(lma)] [ALIGN(section_align)] [SUBALIGN](subsection_align)] {
80    output-section-command
81    ...
82  } [>region] [AT>lma_region] [:phdr ...] [=fillexp] [,]
83
84Output section address
85----------------------
86
87When an *OutputSection* *S* has ``address``, LLD will set sh_addr to ``address``.
88
89The ELF specification says:
90
91> The value of sh_addr must be congruent to 0, modulo the value of sh_addralign.
92
93The presence of ``address`` can cause the condition unsatisfied. LLD will warn.
94GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that
95sh_addr=0 (modulo sh_addralign).
96
97Output section alignment
98------------------------
99
100sh_addralign of an *OutputSection* *S* is the maximum of
101``ALIGN(section_align)`` and the maximum alignment of the input sections in
102*S*.
103
104When an *OutputSection* *S* has both ``address`` and ``ALIGN(section_align)``,
105GNU ld will set sh_addralign to ``ALIGN(section_align)``.
106
107Output section LMA
108------------------
109
110A load address (LMA) can be specified by ``AT(lma)`` or ``AT>lma_region``.
111
112- ``AT(lma)`` specifies the exact load address. If the linker script does not
113  have a PHDRS command, then a new loadable segment will be generated.
114- ``AT>lma_region`` specifies the LMA region. The lack of ``AT>lma_region``
115  means the default region is used. Note, GNU ld propagates the previous LMA
116  memory region when ``address`` is not specified. The LMA is set to the
117  current location of the memory region aligned to the section alignment.
118  If the linker script does not have a PHDRS command, then if
119  ``lma_region`` is different from the ``lma_region`` for
120  the previous OutputSection a new loadable segment will be generated.
121
122The two keywords cannot be specified at the same time.
123
124If neither ``AT(lma)`` nor ``AT>lma_region`` is specified:
125
126- If the previous section is also in the default LMA region, and the two
127  section have the same memory regions, the difference between the LMA and the
128  VMA is computed to be the same as the previous difference.
129- Otherwise, the LMA is set to the VMA.
130
131Overwrite sections
132~~~~~~~~~~~~~~~~~~
133
134An ``OVERWRITE_SECTIONS`` command looks like:
135
136::
137
138  OVERWRITE_SECTIONS {
139    output-section-description
140    output-section-description
141    ...
142  }
143
144Unlike a ``SECTIONS`` command, ``OVERWRITE_SECTIONS``  does not specify a
145section order or suppress the built-in rules.
146
147If a described output section description also appears in a ``SECTIONS``
148command, the ``OVERWRITE_SECTIONS`` command wins; otherwise, the output section
149will be added somewhere following the usual orphan section placement rules.
150
151If a described output section description also appears in an ``INSERT
152[AFTER|BEFORE]`` command, the description will be provided by the
153description in the ``OVERWRITE_SECTIONS`` command while the insert command
154still applies (possibly after orphan section placement). It is recommended to
155leave the brace empty (i.e. ``section : {}``) for the insert command, because
156its description will be ignored anyway.
157